Typically, an imaging spectrometer is composed of an objective or imaging optical module (also called foreoptics), which forms a scene image at an entrance slit of the spectrometer, and a spectral optical module, which receives and collimates the line field of view from the objective, disperses or separates the electromagnetic radiation as a function of wavelength, and images it onto a two-dimensional detector array. Both optical modules can have a variety of different optical forms, and can be reflective, refractive, or a combination of both. Reflective systems are generally recognized to provide advantages in terms of being somewhat easier to construct in large sizes (to provide large apertures), being generally lighter in weight that equivalent refractive systems, and being generally free of chromatic aberrations over a wide spectral bandwidth.
For imaging systems that image electromagnetic radiation from a distant object, the minimum number of optical elements is generally recognized to be three, to provide the minimum number of parameters that are necessary to correct for and/or prevent spherical aberration, coma, astigmatism and field curvature. An optical imaging system composed of three optical elements is often known as a triplet.
Reflective optical triplets are generally constructed such that entering electromagnetic radiation is received on a primary mirror, is reflected from the primary mirror onto a secondary mirror, is reflected from the secondary mirror onto a tertiary mirror, and finally, is focused onto an image plane where an image of the viewed object or scene is formed. Historically, a positive/negative/positive reflective optical triplet can be traced back to the work of Maurice Paul, in 1935 and James G. Baker, in 1945.
Many reflective optical triplets are configured such that all the optical elements lie on the optical axis of the optical system. For example, U.S. Pat. No. 3,460,886 to Rumsey (the “Rumsey '886 patent”) discloses a reflective triplet that is arranged to be “on-axis” in terms of both aperture (i.e., the aperture stop is located on the optical axis) and field of view. This arrangement results in the occlusion of a significant portion of the electromagnetic radiation entering the system from a distant object, a restriction of the field of view of the system, and a constraint on the power distribution between optical elements. In the optical system described in the Rumsey '886 patent, the aperture stop is located on the optical axis and physically on the secondary mirror. In addition, the system has a virtual entrance pupil, is near telecentric, and has rotational symmetry.
Due to the drawbacks associated with a system such as that described in the Rumsey '886 patent, other reflective optical triplets are configured such that the field of view is not along the optical axis of the optical system but entirely to one side of it. For example, U.S. Pat. No. 4,240,707 to Wetherell et al. (the “Wetherell '707 patent”) discloses a reflective optical triplet that is on-axis in aperture, but off-axis in field of view. The reflective optical triplet described in the Wetherell '707 patent has an aperture stop that is on the optical axis and is physically located on the secondary mirror. The entrance pupil to the optical system described in the Wetherell '707 patent is located a large distance behind the optical system, and as such, is virtual. In addition, the system has annular symmetry and is near telecentric.
Other reflective triplets are arranged to be off-axis in both aperture and field of view. For example, U.S. Pat. No. 4,733,955 to Cook (the “Cook '955 patent”) describes a reflective triplet having a real entrance pupil that is located off-axis. The reflective optical triplet in the Cook '955 patent, as a natural result of the real entrance pupil, has a defining front aperture stop coincident with the real entrance pupil. The reflective triplet described in the Cook '955 patent has bilateral symmetry, and a virtual exit pupil. U.S. Pat. No. 8,248,693 to Cook (the “Cook '693 patent”) discloses another example of a reflective triplet that is off-axis in both aperture and field of view. The reflective optical triplet in the Cook '693 patent has bilateral symmetry, a virtual entrance pupil, a real exit pupil, and a rear aperture stop located between the tertiary mirror and the image plane.
In certain spectrometers, a reflective triplet can be used as the spectral optical module. For example, U.S. Pat. No. 5,260,767 to Cook (the “Cook '767 patent”) discloses an all-reflective imaging spectrometer having a three-mirror anastigmat acting as its objective and a reflective triplet with a dispersive element providing the spectral optical module. Similarly, U.S. Pat. No. 7,382,498 to Cook (the “Cook '498 patent”) discloses the use of the non-relayed reflective triplet optical form for the spectral optical module of a spectrometer system. However, in other spectrometers the spectral optical module has a different optical form. For instance, U.S. Pat. No. 9,354,116 to Cook (the “Cook '116 patent”) discloses various examples of the spectral optical module based on four-mirror and five-mirror anastigmat optical forms. Although relatively uncommon, a reflective triplet can also be used as the objective or foreoptics for certain types of spectrometers. For example, U.S. Pat. No. 6,100,974 discloses coupling a reflective triplet of the type disclosed in the Wetherell '707 patent to an Offner-Chrisp-type spectrometer.